100% of the energy becomes heat eventually. For a typical trip, this happens by the trip conclusion. (And also each time the car pauses, such as a stoplight.)

For the trailer-on-hill example, it concludes when the trailer (eventually) is towed or rolls down the hill and comes to a rest from friction.

The weighted trailer is being used like a battery and modifies the situation in the same way as if it were a hybrid car (non-plug-in battery that recharges through regenerative braking and/or directly from the ICE).